Nowadays worldwide desire in diesel automobiles is to reduce particulate matters (hereinafter referred to as PM) in diesel exhaust gas. As a measure for reducing PM, an exhaust emission control device has been equipped for depuration of the exhaust gas.
The exhaust emission control device is provided with, for example, a particulate filter. The particulate filter has a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages; alternate ones of the passages have plugged inlets and the remaining passages with unplugged open inlets have plugged outlets. Only the exhaust gas passing through thin porous walls compartmentalizing the respective passages is discharged downstream while PM in the exhaust gas are captured by inner surfaces of the walls.
PM in the exhaust gas captured by the inner surfaces of the thin porous walls accumulate. Thus, PM require to be properly burnt off for regeneration of the particulate filter before exhaust resistance increases due to clogging.
Various ideas have been proposed since, in the diesel automobile with a normal operation state, the exhaust has little chance to reach a temperature at which PM ignite by themselves. For example, an oxidation catalyst with active species of, for example, Pt and Pd is integrally carried by the particulate filter to facilitate oxidation reaction of the captured PM to thereby lower an ignition temperature. Thus, PM are burnt off at an exhaust temperature lower than that in a structure with no oxidation catalyst.
However, even in the case of the diesel automobile employing the particulate filter integrally carrying the oxidation catalyst, a captured amount of PM may exceed a treated amount in an operation area having a lower exhaust temperature level. Continued operation at such lower exhaust temperature level may hinder sufficient regeneration of the particulate filter, resulting in excessive accumulation of the captured PM in the particulate filter.
Thus, the diesel automobile is provided with a flow-through type oxidation catalyst on an entry side of a particulate filter. Moreover, some vehicles have an after turbo catalyst (hereinafter referred to as ATC) downstream of a turbocharger.
In a step where an accumulated amount of PM has increased, using a fuel injection device, main injection of fuel near a compression upper dead center is followed by post injection at non-ignition timing after the compression upper dead center to thereby add the fuel to the exhaust gas in the exhaust pipe.
As a result, the added fuel undergoes oxidation reaction during passing of hydrocarbon through ATC and the oxidation catalyst. The exhaust gas is heated by resultant reaction heat. Then, the exhaust gas heated increases a catalyst bed temperature of the particulate filter arranged downstream. Thus, PM are burnt off to regenerate the particulate filter.
Advanced engine control is required for regeneration of the particulate filter. Thus, employed as the fuel injection device is, for example, a common rail type high-pressure fuel injection device which can conduct precise multiple injection.